The present invention relates to a plasma processing apparatus which performs a predetermined process by generating a plasma.
In the manufacture of a semiconductor device or flat panel display, plasma processing apparatuses are used often to perform processes such as formation of an oxide film, crystal growth of a semiconductor layer, etching, and ashing. A case wherein a plasma processing apparatus is applied to an etching apparatus will be described. FIG. 9 is a view showing an arrangement of an etching apparatus using a conventional plasma processing apparatus.
A susceptor 521 having a support surface for placing a wafer W thereon and an upper electrode 531 parallel to the support surface of the susceptor 521 are arranged in a process chamber 511. The susceptor 521 also serves as a lower electrode.
Exhaust ports 513 for evacuating the interior of the process chamber 511 to a predetermined vacuum degree are formed in the bottom of the process chamber 511, and a gas supply nozzle 514 for supplying process gases into the process chamber 511 is provided to the side wall of the process chamber 511.
The upper electrode 531 is connected to an RF power supply 534, which outputs an RF power of, e.g., 60 MHz, through a matching circuit 535. When the power supply 534 starts supplying the RF power with a frequency of 60 MHz to the upper electrode 531, an electric field with a frequency of 60 MHz is formed in the space between the upper electrode 531 and susceptor 521. This electric field ionizes the gases supplied from the gas supply nozzle 514 to generate a plasma P. The plasma P is utilized for etching the wafer W placed on the support surface of the susceptor 521.
When performing an etching process, the distribution of the plasma P is preferably not distributed in the entire process chamber 511 but distributed over the support surface of the susceptor 521 with a high density. This is because with this distribution, the etching process can be performed efficiently and etching of the inner wall surface of the process chamber 511 by the plasma P can be suppressed, so that the service life of the process chamber 511 can be prolonged.
In view of this, according to this etching apparatus, a filter 527 formed of an LC series resonance circuit is inserted between the susceptor 521 and ground. The resonance frequency of the filter 527 is set to 60 MHz, which is the same as the frequency of the RF power to be supplied to the upper electrode 531. For example, if L=0.07 μH and C=100 pF, the resonance frequency of the filter 527 can be set to 60 MHz. The frequency characteristics of the filter 527 are as indicated by a solid line in FIG. 10, and the impedance is the minimum when the frequency is 60 MHz.
When the plasma P is generated, however, an ion sheath SH is formed between the plasma bulk and the upper electrode 531 or susceptor 521. An electric field is formed in the layer of the ion sheath SH, and accordingly a new capacitance is generated between the upper electrode 531 and susceptor 521 by the generation of the plasma P. For example, assume that a capacitance of 200 pF is generated by the ion sheath SH. Even when the resonance frequency of the filter 527 is designed at 60 MHz as described above, the frequency characteristics of the first path extending from the upper electrode 531 to reach ground through the susceptor 521 and filter 527 are as indicated by a broken line in FIG. 10, and the resonance frequency of the first path becomes 74 MHz. Hence, even if the filter 527 is designed without considering the influence of the ion sheath SH as in the prior art, the frequency of the RF power cannot cause resonance when the plasma P is generated, so the impedance of the first path cannot be decreased sufficiently. Consequently, the plasma P cannot be sufficiently concentrated on the support surface of the susceptor 521.
When process conditions such as the power value of the RF power to be supplied to the upper electrode 531, the pressure in the process chamber 511, the type and mixing ratio of the process gases, and the like are changed, the capacitance obtained by the ion sheath SH described above also changes. Accordingly, even when the filter 527 is designed with a consideration to the influence of the ion sheath SH formed when a plasma is generated under predetermined process conditions, if the process conditions differ from the predetermined process conditions, the frequency (e.g., 60 MHz) of the RF power cannot cause resonance.
When the etching process is to be performed, the plasma P is preferably concentratedly distributed on the support surface of the susceptor 521, as described above. When the interior of the process chamber 511 is to be cleaned, the plasma P is rather preferably diffused in the entire process chamber 511. In this manner, preferable plasma distribution differs depending on the object of the process. Conventionally, the filter 527 is designed for the etching process, and accordingly its characteristics are fixed. Hence, the interior of the process chamber 511 cannot be cleaned under preferable conditions.
When deposits attaching to the inner wall surface or the like of the process chamber 511 peel off during the etching process and form particles, the particles attach to the wafer W to decrease the yield of elements to be formed on the wafer W. Therefore, desirably no deposit preferably attaches at all to the inner wall surface of the like of the process chamber 511, or if any, they desirably attach stably so they will not peel off during the process. The deposit attaching state, however, changes depending on the process conditions as described above. When the process is performed after changing the process condition, the deposit attaching state changes to form particles, which may decrease the yield.
The above problems arise not only when the plasma processing apparatus is applied to an etching apparatus, but are common among plasma processing apparatuses.